Pyrolytic boron nitride, or chemical vapor deposited (CVD) boron nitride, has been used extensively for the manufacture of refractory shapes. pyrolytic boron nitride shapes have the advantage over sintered boron nitride (BN) shapes in that pyrolytic boron nitride is usually denser and does not contain impurities and unfavorable pores, which are usually found in sintered shapes. Pyrolytic boron nitride shapes are often used as crucibles for melting semiconductor materials, and for high-temperature jigs and insulators. While, pyrolytic boron nitride shapes are satisfactory for many of these refractory uses, boron nitride is subject to physical change under conditions of high mechanical pressure and high temperature. Therefore, there is a continuing need for a refractory which is more stable under high stress and high temperature conditions than is conventional pyrolytic boron nitride.
Another use for pyrolytic boron nitride is as a coating upon graphite shapes to, for example, increase high temperature oxidation resistance, and prevent carbon contamination in semiconductor applications. A problem with boron-nitride coated graphite shapes is that pyrolytic boron nitride has a significantly different coefficient of thermal expansion (CTE) than graphite. This can cause failure of the coating under thermal cycling and high temperature conditions.
Matsuda, et al. in "Synthesis and structure of Chemically vapour-deposited boron nitride" Journal of Materials Science 21 (1986) 649-658, disclose the formation of boron nitride by chemical vapor deposition under conditions to form boron nitride with a mixed morphology, i.e. with regions of turbostratic boron nitride (t-BN), which are lacking in three dimensional ordering, and regions of hexagonal boron nitride (h-BN). Under SEM photography, the surface of a pyrolytic boron nitride plate which was predominately h-BN was disclosed as being composed of pyramidal pentagonal facets. The mixed t-BN/h-BN composition is disclosed as having the same structure as pyrolytic boron nitride commercially available from Union Carbide Corp.
Kurdymov, A. V. et al. in "Twinning in graphite-like boron nitride," Sov. phys. Crystallogr , v.21, p.229-31, No. 2, Mar.-Apr., 1976, disclose twinning in graphite-like boron nitride crystals.
Twinning, as disclosed for example in Kurdymov, is the rotation of a part of the crystal lattice such that the rotated part has a basal plane tilted from the basal plane of another portion the crystal. Twinning may occur more than once in a crystallite. Thus, a boron nitride crystallite may comprise several regions of hexagonal boron nitride with basal planes intersecting and tilted relative to those of neighboring regions. Due to the tilted basal planes, the twinned crystallites often occur with pyramidal pentagonal facets, as disclosed, for example, in Matsuda, et al.